Heat-treating magnesium alloys



meted oeeso, 194sassaizo omen HEAT-TREATING MAGNESIUM ALLOYS Robert 8. Bush, Midland, Micln, assignor to The Dow Chemical Company, corporation of Michigan Application December 2:, i943, Serial No. 515,420

5 Claims. '(Cl. 148-213) No Drawing.

This invention relates to the heat-treatment of magnesium-base alloys. It particularly concerns a preliminary sequence for minimizing germination during solution-heat-treatment of magnesium-base ,alloy castings.

During the heat-treatment of metals, there is sometimes observed the phenomenonof germination, or sporadic uncontrolled grain growth.

(See, for example, the discussion in The Science of Metals," p. 133 ff., Jefiries and Archer, McGraw-Hill, New York, 1924.) Thus, after prolonged solution-heat-treatment of certain magnesium-base alloy castings, local areas of the metal may exhibit, instead of the usual large number of small grains, one or a few relatively enormous grains having a' diameter of a tenth of an inch or more; single grains more than half an inch in size are occasionally seen. Inasmuch as the occurrence of such macro-crystalline areas may so weaken themetal as to render it unlit for service, the avoidance of germination during Millar Mich I usual 0.1 to 0.2 per cent manganese is absent, and to a lesser extent those alloys containing up to 4 percent of zinc, as well as aluminum and manganese, appear to be particularly susceptible to sporadic grain growth. For this reason, the preheating. according to the present inven-' tion is most commonly used as a step prior to the solution-heat-treatment of magnesium-alu minum-z inc alloys of the type mentioned.

Among the commercial magnesium-base alloys to which the invention is most frequently applied are: Dowmetal C, containing from 8.3 to 9.7 per cent aluminum and.1.'7 to 2.3 per cent zinc and about 0.1 per cent manganese; Dowmetal G, containing from 9 to 11 per cent aluminum, up to 0.3 per cent zinc and about 0.1 per cent manganese; and Dowmetal R, containing from 8.3 to

9.7 per cent aluminum, up to 1.0 per cent zinc,

heat-treatmentof magnesium alloys is of great 7 practical importance.

According to the present invention, uncontrolled grain growth during the solution-heattreatment of a heat-treatable magnesium-base alloy characterized by a tendency toward germination may be largely prevented by a pre-treatment which comprises pro-heating the metal for a period not exceeding about 30 minutes at a temperature substantially, above its maximum solution-heat-treating temperature. This preheating appears to alter the ultimate structure of the metal, possibly either by equalizing the growth tendencies of the crystals already present in the metal or by developing new centers of nucleation at a common energy level, so that excessive grain growth does not occur during subsequent solution-heat-treatment.

In so far as known, the process of the invention may he successfully applied to articles formed of any heat-treatable magnesium-base alloy which tends to undergo germination on prolonged heating. In this regard, however, it may be noted that the tendency toward uncontrolled crystal growth is most common with-cast metal, especially metal which has been locally strained by drastic chilling in the mold. The practice of superheating magnesium alloys to effect grain refinement prior to also seriously aggravates, and may even initiate, germination during subsequent heat-treatment of the casting. Commercial magnesium alloys containing from 8 to 14 per cent aluminum as the major alloying element, especially those in which the and about 0.2 per cent manganese, magnesium constituting the balance of the alloy in each case.

The pre-heating temperature of the invention is, as already stated, substantially above the maximum solution-heat-treating temperature for the particular alloy involved, but is, of course, also below the melting point of the article. The preheating temperature, unlike that of solution- .heat-treatment, may be considerably above the temperature of fusion of the lowest melting constituent present in the metal, but the local fusion which may be thus produced does not adversely affect the properties of the metal since the duration of the pre-heating is so short. With magnesium-base alloys containing 8 to 14 percent aluminum, and up to; 4 per cent zinc, the preheating temperature is in the range 800 to 900 F., but above the normal solution-heat-treating temperature, 850 F. being ordinarily preferred.

For magnesium alloys containing aluminum and little or no zinc, e. g. Dowmetal G, pre-heating temperatures begin at about 825 F. v

The pre-heating step is best accomplished in a standard furnace provided with a protective atmosphere, or in a molten salt bath. For best results, the temperature of the metal should be raised as rapidly as possible, most conveniently by inserting the article to be pre-heated into a "furnace which is already at the required temperature. After the brief pro-heating according to the invention, the article is cooled to a temperature sufllciently low to effect solidification of any locally fused regions produced by the pre-heating,

- e. g. to below about 640 F. in the case of alloys containing aluminum as the major alloying element. It is then subjected to solution-heattreatment in conventional manner, 1. e. by heating slowly to a temperature above the temperature limit of solid solubility in magnesium of the existing concentration of the major alloying element and maintaining it at such temperature for a time sufficient to effect solution of any major second phase present. For magnesium-aluminum-zinc alloys of the type described, e. g. Dowmetal and R, solution-heat-treating temper-- atures of 715 to 785 F. and times of 16 to 25v hours are ordinarily'used, and for alloys of the type of Dowmetal G the range is 780 to 820 F.

The heating from below 640' F. into the solution heat-treating temperature range requires 1 to 2 hours. p

The manner of cooling the pre-heated article prior to solution-heat-treatment is not critical, it being satisfactory merely to transfer the article from the pre-heatihg furnace to the cooler heattreating furnace.

-: -The following example will further illustrate the invention.

1 Example fore solution-heat-treatment, in accordance with the invention, showed no germination after solution-heat-treatment at 770 F. for 18 hours.

The foregoing description is illustrative rather than strictly limitative, the invention being coextensive in scope with the following claims.

What is claimed is:

1. An improved procedure for producing in an ungerminated condition solution-heat-treated cast articles formed of a heat-treatable magnesium-base alloy characterized by a tendency to.

undergo germination on prolonged heat-treatment, which comprises pre-heating the article for aperiod not exceeding about 30 minutes at a assarso I temperature substantially above the fusion point of the lowest melting constituent of the alloy, and thereafter cooling the article and subjecting it to solution-heattreatment.

2. An improved procedure for producing in an .ungermlnated condition after solution-heattreatment castings poured from a superheated heat-treatable magnesium-base alloy containing from 8 to 14 per cent aluminum and up to 4 per cent zinc as the major alloying elements, which comprises pre-heating the casting for a period of 5 to minutes at a temperature in the range 800 to 900 F., and then coolingthe casting to below about 640 F., and subjecting it to solutionheat-treatment at a temperature in the range test panels which were pre-heated for 15 minutes at 850 F. and then cooled to below 640 F. be-

v mination' which comprises pre-heating the cast-' 3. An improved procedure for producing in an ungerminated condition after solution-heattreatnient castings poured from a superheated magnesium-base alloy containing from 8.3 to,9.'l per cent aluminum and from 1.7 to 2.3 per cent zinc as the major alloying elements which comprises pre-heating the casting at a temperature of about 850 I". for from 15 to 30 minutes, and

then cooling the casting to below about "640 1''. and subjecting it to solution-heat-treatment at a'temperature in the range 760 to 780 1".

4. Ina process of solution-heat-treating casts;

ings formed of a heat-treatable magnesium-base alloy characterized by a tendency to undergo germination. on prolonged heating; a pre-treatment for minimizing germination which, comprises heating the casting at a temperature substantially above the fusion point of the lowest melting constituent of the alloy for a period not exceeding about 30 minutes prior tosubjecting ittosolution-heat-treatment. i V

5. In a process of aolution-heat-treating castings poured from a superheated magnesium-base alloy containing from 8 to 14 per cent aluminum. and up to 4 per cent zinc as the major alloying elements, a pre-treatment for minimizing gering at a temperature inthe range 800 to 9 00 l". fora period of 5 to 30 minutes, and thereafter cooling it to below about 640 F. prior to subjecting it to solution-heat-treatment.

; ROBERT B. BUBK. 

